(1) Field of the Invention
The invention relates to a control circuit for an amplifier with an output-controlled automatic gain control for a hearing aid, comprising an output amplifier stage, a magnetic earpiece which has a working winding connected between the output of the output amplifier stage and the operating voltage, and a control loop having a recitifier, for automatic gain control, which control loop is connected between the output of the output amplifier stage and the input of the output amplifier stage or a preamplifier stage if provided.
Hearing difficulties (deafness) may be roughly divided into two different groups, more specifically what is known as sound conduction deafness (middle ear deafness) and what is referred to as acoustic perception deafness (inner ear deafness). However, while in the case of sound conduction deafness, both the threshold of audibility and also the pain threshold are generally shifted upwardly by about the same amount and therefore it is possible to achieve good correction with greater or lesser amplification of the sound signal supplied, acoustic perception deafness often only involves upward displacement of the threshold of audibility, while the pain threshold is at the same level or even at lower values than in the case of a healthy ear.
(2) Description of the Prior Art
In order to take account of the dynamic range of the acoustic signals which occurs in practice, dynamic compression is always provided in hearing aids for inner ear deafness. A number of circuits have been developed for that purpose. One of those circuits which has proved successful in practice is automatic gain control.
In that circuit, a part of the output voltage is taken off at the output of the output amplifier of the hearing aid, that is to say, at the earpiece itself, rectified and filtered. The control voltage which is obtained in that way is generally fed to a preamplifier stage where it provides for gain control which is inversely proportional to the magnitude of the control voltage. This is effected, for example, by displacing the working point of the controlled stage or by a variable controlled negative feedback. The circuit operates like a dynamic limiter, that is to say, when high amplitudes occur at the output, the control means comes into operation and suitably reduces the gain of the input stage.
A serious disadvantage of that known circuit is that the signal ac voltage which occurs at the earpiece is generally only about 300-500 mV under the conditions obtaining in the hearing aid. The level of efficiency of rectification is therefore low and the reduction in gain when the control means comes into operation remains within restricted limits. It has also been proposed that, instead of a single diode, use may be made of full-wave rectification or voltage doubling in order to obtain a stronger control signal. However, it is then often no longer possible for the components which are additionally required for that purpose to be disposed in a hearing aid without increasing its size. The attempt to replace normal silicon diodes by Schottky diodes is also unsuccessful as both behave in a very similar fashion with the small currents which occur in this context. Finally, the use of germanium diodes which have a noticeably lower forward voltage encounters serious difficulties as those diodes are not produced in miniaturised form and are therefore generally unsuitable for use in a hearing aid.